An endoprosthesis is an artificial device that is placed inside the body, and more particularly, within an anatomical lumen. An anatomical lumen is a cavity of a tubular organ, examples of which include without limitation a urinary tract, bile duct, fallopian tube, esophagus, and arterial or venous vasculature. Stents are a type of endoprosthesis which are often used in the treatment of atherosclerotic stenosis in blood vessels. Stenosis refers to a narrowing or constriction of the diameter of an anatomical lumen or orifice. Stents can reinforce the walls of the anatomical lumen and prevent a reoccurrence of stenosis.
The process of locating a region that may require treatment with an endoprosthesis often involves obtaining an x-ray image of an anatomical lumen. A contrast agent, which contains a radiopaque substance such as iodine is injected into the anatomical lumen. “Radiopaque” refers to the ability of a substance to absorb x-rays. The x-ray image allows a physician to identify a potential treatment region at which a stent can be delivered and deployed. Delivery involves transporting the stent through the anatomical lumen, which can be tortuous and require the stent to bend in order to pass through sharp bends. Deployment involves expanding the stent within the treatment region so that it engages and supports the walls of the anatomical lumen.
In addition to meeting the mechanical requirements for delivery and deployment, it is desirable for a stent to be radiopaque, or fluoroscopically visible under x-rays. Accurate stent placement is facilitated by real time visualization of the stent during delivery and deployment to allow for precise placement. This can be accomplished by fluoroscopy or similar x-ray visualization procedures. For a stent to be fluoroscopically visible it must be more absorptive of x-rays than the surrounding tissue.
Stents made of bioresorbable polymers have been developed to allow for improved healing of the anatomical lumen. Examples of bioresorbable polymer stents include those described in U.S. Pat. No. 8,002,817 to Limon, U.S. Pat. No. 8,303,644 to Lord, and U.S. Pat. No. 8,388,673 to Yang, which are incorporated herein by reference. As used herein, “bioresorbable” refers to a material capable being completely eroded, degraded (either biodegraded and/or chemically degraded), and/or absorbed when exposed to bodily fluids (such as blood or other fluid); and can be gradually resorbed, absorbed and/or eliminated by the body. Other terms such as biodegradable, bioabsorbable, and bioerodible may be found in the literature, and while these terms have specific definitions, they are often used interchangeably. The term “biostable” refers to a material that is not bioresorbable.
A drawback of bioresorbable polymers (and polymers generally composed of carbon, hydrogen, oxygen, and nitrogen) is that they are radiolucent with no radiopacity. Biodegradable polymers tend to have x-ray absorption similar to body tissue. This is addressed by attaching radiopaque markers to structural elements of the stent. Attachment entails a number of challenges. First, radiopaque markers are often very tiny which makes insertion into a stent difficult. Second, polymeric stent struts undergo significant deformation during delivery and deployment, during which the radiopaque marker must remain attached to the stent. Although the use bioresorbable materials typically means that material thicknesses are greater than those required of metals, improvements in bioresorbable stent design and manufacturing are allowing for stent struts with reduced thickness. For example, the thickness of a bioresorbable stent strut could be as low as 0.003 inch (0.075 mm). With reduced thickness, there is less volume in the stent strut to ensure that the radiopaque marker remains in place. Thus, it is important to maximize engagement between the radiopaque marker and the endoprosthesis.
Accordingly there is a continuing need to for a system and method of attaching radiopaque markers to help maximize engagement to the endoprosthesis. Also, there is a continuing need to for a system and method of attaching radiopaque markers with greater efficacy and uniformity.